At that point the acid process ceases, but in the basic process the blast of air is continued and the phosphorus is oxidized and passes into the slag. The slag therefore contains a considerable percentage of phosphorus and this makes it valuable as a fertilizer. The demand for it is unlimited and the revenue derived from it is a very important matter to all plants using this process. The cost of labor, however, and the greater waste and diminished output of a basic Bessemer render this process out of the question except where suitable pig-iron can be had at a much lower price than iron fit for the acid process. In the United States this condition does not exist and there is no plant in operation in this country.
The final operation of adding spiegel iron or ferromanganese is conducted in practically the same way in the basic Bessemer vessel, as has already been described in the account of the acid process.
THE OPEN-HEARTH FURNACE.
An open-hearth furnace really means a furnace having a hearth exposed to the flame, so that any piece of steel or other material placed upon the hearth is exposed openly to the action of the burning gases. The term has- been narrowed by custom to denote such a furnace where steel is melted. A furnace for this purpose must be regenerative in order to get the requisite intense temperature. Regenerative furnaces are also used very generally for heating steel in rolling mills, but they are not called open-hearth furnaces except when the steel is actually melted.
By a regenerative furnace is meant one in which the heat carried away in the stack gases is used to warm the air and gas before they enter the furnace. Strictly speaking, a furnace would be regenerative if air pipes were put into the stack and the air blast were passed through these pipes. But by custom the term means only a furnace which is heated by gas, and where both gas and air are heated before they enter the furnace by being passed through chambers filled with bricks loosely laid, these bricks having previously been heated by the waste gases. By having two sets of chambers, one set can be used to absorb the heat in the waste products and the other set to warm the incoming gases. By proper systems of reversing valves these two sets of chambers can be used •alternately for each purpose, and in this way the gas and air are • heated to a yellow heat before they unite, and it is quite evident that yellow-hot air and yellow-hot gas will give a very intense heat.